This invention relates to metal chromate pigment compositions and, more particularly, to substantially non-dusting thermally stable lead chromate pigment compositions and their preparation.
Metal chromate pigments have been known and widely used for many years, and are available in a broad range of shades from very green shade yellows to yellowish reds. Of the various metal chromate pigments, e.g., strontium chromate, zinc chromate, and lead chromate, lead chromate is the most widely used. These chromate pigments are relatively inexpensive to manufacture and have generally good tinctorial properties.
Thermal stability, i.e., resistance to darkening at elevated temperatures in contact with thermoplastics, and chemical resistance of lead chromate pigments have been improved markedly by the application of coatings of hydrous oxides such as silica and alumina to the base or uncoated metal chromate pigment, as described for example in Linton U.S. Pat. No. 3,370,971. To improve the mechanical strength of the coated metal chromate pigments, alkaline earth metal salts of rosin acids and of long-chain fatty acids have been applied to the pigments, as described in Linton U.S. Pat. No. 3,470,007.
Metal chromate pigments, though manufactured in aqueous systems, are commonly dried to a powder prior to shipment and use in coating compositions. Packaging, pouring and otherwise handling the dry pigment can create a dusting problem in the immediate work environment. If the dust level is sufficiently high, special protective equipment may be necessary to insure proper worker safety. With increased industry and government concern over protection of the environment and worker health, it is highly desirable to have a dry chromate pigment which is substantially dust-free. The resulting elimination of the need for special protective equipment can cut the cost of pigment to the manufacturer and user and insure worker safety with less monitoring and inconvenience than is currently employed.
In addition to dusting problems, the thermal stability of conventional lead chromate pigments, while improved, is not sufficiently high for particularly demanding applications in thermoplastics. This is particularly true under conditions of exceptionally high temperature processing.
This invention provides for substantially dust-free lead chromate pigment which exhibits all the desirable properties common to conventional lead chromate pigments and exhibits much higher thermal stability.